The deposition of materials from a gaseous phase onto a substrate, and in particular onto a semiconductor substrate, is well known. One class of well known deposition techniques employs photolithographically produced masks for generating patterned deposits for use in fabricating an integrated semiconductor device. These techniques require multistep processing on a microscopic level. Typically, these techniques provide a resolution which is limited by the resolution of the masks, the control and registration by which the masks can be positioned (and repositioned) during the manufacturing process, and the subsequent etching and deposition techniques. Thus while the masks themselves are usually generated optically and have a high spatial resolution, the spatial resolution of the final product is limited by the ability to register the mask, control the optical mask making process, and control the etching and deposition procedures employed therewith. In addition, the masks themselves have inherent defects (such as pin holes, "mousenips", and other (faults) which must be corrected.
Other methods for forming thin film structures by deposition from the gaseous phase without the use of masks are also well known. Thus conventional chemical vapor deposition, a maskless process, relies on the use of a heated substrate to produce localized pyrolysis of one or more of the carrier gases with the subsequent formation of, for example, a semiconductor film on the substrate. This process is clearly disadvantageous at least because the heated substrate and film, when cooled, generate stresses which can seriously affect the electrical and mechanical properties of the device. These stresses result from a generally different coefficient of expansion for the deposit when compared to the substrate. In addition, conventional chemical vapor deposition processes are not used for producing small structures.
Others have recognized the need for producing controlled deposits on a substrate, and in particular on a semiconductor substrate surface; and they have employed photolytic processes for fabricating thin film patterns. Thus, for example, White U.S. Pat. No. 3,271,180, describes a method for forming a thin film of material in a geometric pattern upon a rigid substrate by introducing, into a chamber containing the substrate, a vapor of material to be deposited and then controlling the pressure of the vapor to form an adsorbed layer of the material on the substrate surface. Only the adsorbed layer is then photolytically reacted by incident light of a first predetermined wavelength. (The vapor is capable of being photolytically reacted in the gas phase only by incident light of a second predetermined wavelength, whereas the first (absorbed layer reacting) wavelength is greater than the reaction wavelength of the vapor (the second wavelength) in the gas phase.) Thus according to White it is only the vapor which has been adsorbed, i.e. the layer in the surface, which is reacted to provide the desired deposit on the substrate surface. White employs an incoherent light source and is limited in brightness and resolution by the losses due to optical focusing (which controls the resolution of the deposit) and the losses due to the filtering requirements to obtain a substantially monochromatic incident light at the first predetermined wavelength (resulting in reduced brightness). The filtering requirement reduces the incident light brightness so that direct writing of one micrometer lines, at practical rates, cannot be obtained.
Other workers in this field have employed photolytic decomposition on, for example, silicon dioxide by exposing the surface to a condensable vapor, cooling the surface to a temperature below the dew point of the vapor to form a thin liquid film thereon, and exposing at least a portion of the interface between the liquid film and the surface to an activating radiation to cause formation of a surface-imcompatible specie for etching the silicon dioxide surface in the illuminated areas. This is described for example by Schaefer, U.S. Pat. No. 3,494,768. This method thus employs a decomposable liquid on a solid surface and the disclosed sources, like White above, are incoherent sources, for example high pressure mercury lamps.
Other workers in the semiconductor field recognized the need for developing alternate methods of depositing surface-incompatible materials, etchants, from the gas phase onto a semiconductor surface. Thus, Hall, U.S. Pat. No. 2,841,477, describes a relatively broad area method for etching metals and semiconductors by using photochemically activated gases to generate surface-incompatible compositions in the gas phase. Hall introduces an otherwise chemically inactive photolyzable gas (or gases) into a chamber in which the surface of, for example, a semiconductor crystal is exposed. When exposed to his incoherent ultraviolet radiation, which need not be focused, the initially inactive gas produces free radicals which are capable of depositing upon and etching the specimen semiconductor crystal surface. Hall discloses several alternate methods for masking or "concentrating" the incoherent incident light upon the specimen to be etched. In one particular embodiment, Hall employs a convex lens to focus the incident incoherent light substantially upon the semiconductor surface to effect formation of a pit, and in other embodiments he uses "light" shields to selectively block portions of the incident radiation and thereby selectively etch portions of the semiconductor surface. In all embodiments, photodecomposition can take place whenever the incoherent ultraviolet radiation is present in the chamber, that is, whether the incident energy is focused or not, close to or distant from the substrate surface.
It is therefore an object of the present invention to provide an improved method and apparatus for producing, without the use of masks, a thin film in a selected pattern on a substrate surface. Other objects of the invention are providing high resolution, providing a more controlled, yet high deposition rate, composition, and structure in a maskless environment, minimizing unwanted diffusion of material within the substrate during processing, having a minimum cost, and resulting in a reliable manufacturing process and product. Yet further objects of the invention are an apparatus and method which can be employed with a complex substrate in a dry state and which operates at low temperatures. A yet further object is to provide a maskless apparatus and method having a sufficiently rapid direct writing rate to effect practical writing of one micrometer size features.